A white light source is generally provided by mixing light sources of different wavelengths. For example, a conventional white light source can be realized by mixing red light, green light and blue light with a suitable intensity ratio. Alternatively, the white light source can be realized by mixing yellow light and blue light with a suitable intensity ratio. The conventional method for manufacturing a white light source can be summarized as following:
In a first example of a prior art of white light source, three LED dies based on InGaAlP, InGaN and GaP are packaged into a lamp and emit red light, blue light and green light, respectively. The light emitted from the lamp can be mixed by a lens to provide white light.
In a second example of a prior art of white light source, two LED dies based on InGaN and AlInGaP or GaP emit blue light and yellowish-green light. The blue light and yellowish-green light are mixed to provide white light. The white light sources according to above-mentioned two approaches have an efficiency of 20 lm/W.
A third example of a prior art of white light source is proposed by Nichia Chemical Co., in which an InGaN-based blue LED and a yellow YAG phosphor are used to provide the white light source. This white light source requires a uni-color LED to provide white light with an efficiency of 20 lm/W. Moreover, the phosphor is a mature art and commercially available.
A fourth example of a prior art of white light source is proposed by Sumitomo Electric Industries Ltd., and uses a white-light LED based on ZnSe. A CdZnSe thin film is formed on the surface of a ZnSe crystalline substrate. The CdZnSe thin film emits blue light and the ZnSe crystalline substrate emits yellow light after receiving the blue light from the CdZnSe thin film. The blue light and the yellow light are mixed to provide white light. In this approach, only one LED chip is required and the operation voltage thereof is 2.7 V, less than 3.5 V operation voltage of the GaN based LED. Moreover, no phosphor is required.
In a fifth approach to provide white light source, an ultra-violet LED is used to excite a plurality of phosphors such that the phosphors luminesce lights of different colors for mixing into a white light.
In first and second examples of prior art white light sources, LEDs for multiple colors are required. The color of the white light source is distorted if one of the LEDs malfunctions. Moreover, the driving voltages for LEDs of different colors are also different; this complicates the design of driving circuit.
The third example of a prior art white light source employs a complementary color to achieve a white light. However, the white light produced in this way lacks uniform spectral distribution (especially in 40 nm–700 nm) as exists in natural white light such as sunlight. The white light thus produced has relative chroma, which is, even if indistinguishable to human eyes, differentiable to an instrument such as a camera. Therefore, the color rendering property and reproduction ability are not satisfactory and this white light source is used mainly for lighting.
The fourth example of a prior art white light source has the drawbacks of a low luminescent efficiency (only 8 lm/W) and a short lifetime about 8000 hours.
In the fifth example of a prior art white light source, three phosphors are preferably used to emit three different colors to enhance the color rendering property thereof. However, the phosphors should be prudently chosen to have an absorption band that matches the wavelength of the excitation radiation. Moreover, the phosphors should have compatible absorption coefficients and quantum efficiency to provide white light of high quality. These requirements place a strict constraint on the materials of the phosphors. More seriously, the equation governing the mixed color is nonlinear and the color evolution contour is 2D instead of 1D. The mix ratio is thus hard to optimize.